Zinc powder is used for a variety of purposes in the chemical and metallurgical industries. One chemical use is in various reductions (in which the zinc is oxidized to Zn.sup.2+, i.e., converted to a zinc halide or oxide, etc.). Conventional wisdom is to the effect that the purer the zinc, the more effective or "active" it is as a reductant.
Zinc powders (including "dusts") are prepared commercially by electrolytic deposition from neutral, weakly acid or basic solutions of zinc salts--most notably, solutions derived from ores which include zinc minerals. Deposition of the zinc in a form which can readily be removed and converted to a powder is an obvious desideratum. To this end, the use of various additives, including ions of foreign metals, has been tried.
It is taught in Electrodeposition of Metal Powders, A. Calusaru, Ch XVI; Elsevier Pub. (1979), Materials Science Monograph 3, that some metal ions do favor deposition of zinc in the form of a sponge (which can be readily converted to a powder) but that the activity of the zinc is lowered by inclusion of the added metals (as co-electrolysis products).
It is further taught that zinc is easily deposited from weakly acid solutions in sponge form at low current densities (0.4 A/dm.sup.2 =0.0025 A/in.sup.2) but formation of compact deposits is favored by higher current densities.
Also, according to the latter reference, powdery zinc deposits may be obtained (at low current densities) by electrolysis of alkaline solutions of zinc (as zincates, that is) without the use of additives and the zinc produced is more active than zinc produced from neutral or acidic solutions.
Zinc powders of several particle size ranges are listed in the 1983/84 Alfa Catalog (Morton Thiokol, Inc.). Considering only metals, the zinc contents of these powders range from 99.9 to 99.9995%. In other words, the maximum proportion of the total metals present which could be lead range from 5 to 1000 ppm. It is believed that the latter figure is typically a maximum for prior art zinc in powder form.
There is room for improvement in the production of zinc powders, in at least two aspects. If higher current densities could be attained in the productions of zinc powders under acid conditions, acidic ore leaches could be better utilized for this purpose; also waste streams, such as acidic ZnCl.sub.2 solutions formed in reductions of C-Cl groups with zinc, could be recycled economically. If the zinc powders could be made more selective as reductants for polyhalo compounds, such reductions could be carried out more efficiently and with less co-generation of over-reduction products. This latter improvement is particularly to be desired for production of 2,3,5,6-tetrachloropyridine--an important commercial intermediate--from pentachloropyridine.